A minstrel is a wandering musician, though they may also be hired on for permanent positions as well.
"Once upon a time" is a trade joke amongst minstrels.
Frankie of Avalon was a minstrel, but not a very good one.
The bard Shallan was offered the position of court minstrel to Daventry, but was undecided, as it was not his way to stay in one place for long. He at least stayed long enough to finish his accounts of Graham's adventures to save the world from the endless winter, and perform it for the castle.